Production of fire resistant laminates

ABSTRACT

Process for the production of an intumescent fire resistant layer by drying a waterglass solution on the surface of a glass substrate are carried out in the presence of a salt of a carbonic acid or an α-hydroxy carboxylic acid such as potassium citrate. The drying is preferably carried out at a rate which reduces of pH of the solution by no more than 2 units during an initial drying period of 5 hours. The resulting interlayers provide improved fire resistance.

BACKGROUND OF THE INVENTION

This invention relates to novel methods for the production of waterglasscompositions, to the compositions produced by such methods and to fireresistant glass laminates comprising such compositions as an intumescentlayer between two opposed panes of glass.

Glass laminates incorporating an intumescent inorganic layer sandwichedbetween two opposed panes of glass are known and are sold under theTrade Marks PYROSTOP and PYRODUR by the Pilkington group of companies.When such laminates are exposed to a fire the inorganic layer intumescesand expands to form a foam. The foam provides a thermally insulatinglayer which protects the pane of glass remote from the fire so that thestructural integrity of the glass unit is maintained and therebypresents a barrier to the propagation of the fire. The insulatingproperties of the foam layer reduce the amount of heat transmittedthrough the layer and thereby reduce the risk of combustion of materialson the non-fire side of the glass unit. Glass laminates incorporatingsuch inorganic layers have been successfully used as fire resistantglass structures. Such laminates may comprise more than two glass panessandwiching more than one inorganic layer. Depending upon the stringencyof the relevant regulation laminates comprising as many as eightinorganic layers,have been used. The multi-layered laminates arerelatively thick and correspondingly expensive.

The intumescent inorganic layer is normally formed mainly from a sodiumsilicate waterglass or a mixture thereof with a potassium silicatewaterglass. In addition the layer may comprise a minor quantity of apolyhydric organic compound such as a glycol, glycerine and itsderivatives or a sugar. The inorganic layer is normally formed bypreparing an aqueous solution of the waterglass, applying the solutionto one of the panes of glass and drying the excess water so as to formthe inorganic layer. Typically the inorganic layer will compriseapproximately 20% by weight of water (including water of hydration) andvarying amounts of Na₂SiO₂, Na₆Si₂O₇ and Na₂Si₃O₇. The waterglasssolutions and the inorganic layer may contain varying ratios of Na₂O toSiO₂ and may also comprise potassium containing species by introductionof K₂O as an alternative to the Na₂O.

The drying step must be carried out under carefully controlledconditions if an optically clear layer having useful intumescentproperties is to be obtained. Typically the drying is carried out athigh humidity and over a long period say from 12 to 24 hours. Thislengthy process adds to the cost of the manufacturing process andreduces the productivity of the production facility.

U.S. Pat. No. 4,190,698 discloses glass laminates in which theinterlayer is a silicate waterglass and proposes a variety of adjuvantsto the waterglass including glycerine, ethylene glycol, sorbitol,glucose, starch, sodium phosphate, sodium aluminate aluminium phosphate,borax, boric acid and colloidal silica. These adjuvants are stated toimprove the fire resistant performance of the interlayer.

The adjuvant which is most commonly added to a silicate waterglass toform an interlayer having advantageous properties is glycerol. Thepresence of glycerol reduces the cracking of the interlayer during thedrying process and thereby facilitates the formation of a clear driedinterlayer.

The fire resistance of these glazings comprising these interlayersdepends on part upon the integrity of the glazing being maintained foras long as possible even whilst the glass panes are cracking under thestress. In some instances the glass cracks in a catastrophic fashion andthe glazing may fail the particular test as a result. We have observedthat these failures may be associated with the formation of a relativelycoarse foam when the interlayer is heated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have now discovered that the addition of a salt of a carbonic acid orof an α-hydroxy carboxylic acid to the waterglass solution prior to thedrying step leads to the production of an intumescent layer having animproved fire resistant performance. These layers produce a finer foamon exposure to flame and applicants believe that this is indicative ofimproved fire resistant performance. The presence of the salt may alsoenable the drying process to be carried out over a shorter period oftime without any detrimental effect upon the quality of the product thusimproving the productivity of the production facility.

From one aspect this invention provides a process for producing aninorganic intumescent layer upon the surface of a glass substrate whichcomprises spreading a solution of an alkali metal waterglass upon thesurface of the glass and evaporating water from that solution until theinorganic layer is formed wherein the waterglass solution comprises analkali metal salt of a carbonic acid or α-hydroxy carboxylic acid. Thesalt which is employed will preferably be one which does notsignificantly detract from the optical properties of the intumescentlayer or from the fire resistant performance thereof.

Examples of useful salts include the metal salts and especially thealkali metal salts of α-hydroxy carboxylic acids such as citric acid,malic acid, tartaric acid, glycollic acid and lactic acid have all beenfound to be useful in the processes of this invention. Salts of weakinorganic acids such as the alkali metal carbonates and bicarbonateshave been found to be useful.

The quantity of salts required in order to be effective will vary withthe choice of buffering agent and with the composition of the waterglasssolution. Generally the waterglass solution may comprise from 0.1 to 2.0preferably from 0.5 to 1.0% by weight of the salt (prior to drying). Theoptimum quantity of any particular salt may be determined by empiricalmeans. The use of an excessive quantity of any particular salt can leadto the formation of a dried intumescent layer having inferior fireresistant properties compared to the known systems or which isunacceptably hazy.

The alkali metal silicate waterglass is preferably a sodium silicatewaterglass. The preferred waterglasses for use in the compositing of thepresent invention are-those wherein the weight ratio of S_(i)O₂:Na₂O isat least 2:1 more preferably those wherein the weight ratio is at least2.5:1 and most preferably those wherein the weight ratio is at least2.85:1.

Alkali metal silicate waterglass other than sodium silicate waterglassesin particular potassium silicate and lithium silicate waterglasses mayalso be useful in the compositions of this invention. Potassium silicatewaterglasses wherein the weight ratio S_(i)O₂:K₂O is in the range 1.4:1to 2.1:1 are especially useful. Compositions which comprise a mixture ofa sodium silicate based waterglass and a potassium silicate waterglassmay also be useful.

The waterglass solutions which are spread onto the glass surface duringthe processes of this invention will generally have a pH in the range10.0 to 12.0 more preferably in the range 10.5 to 11.8. Typically thesesolutions will comprise from 50 to 70% by weight of water. At the end ofthe drying process the water content of the inorganic intumescent layeris from 15 to 26% by weight more preferably from 18 to 22% by weight.The effective pH of the solution will reduce in line with theproportional decrease in ionic mobility during the drying process. Asthe water is evaporated and the layer begins to solidify the pH becomesa less meaningful measure of the properties of the layer.

In the preferred embodiments of the invention we have discovered thatthe pH of the solution should fall by no more than 2 units of pH over aninitial period of 5 hours. during the drying step. Over this period thewater content of the waterglass solution is preferably reduced from say60% to a value in the range 30 to 35% by weight. In the preferredembodiments of this invention this initial part of the drying processwill be carried out under controlled conditions and the rate of waterloss will be broadly the same or longer than that currently used to drywaterglass solutions of the same depth in the absence of the salts ofthis invention. However in the processes of this invention carried outwe prefer that the later part of the drying process be completed in ashorter time. The overall drying time in a typical prior art process maybe anything from 12 to 24 hours. In the preferred processes of thisinvention the overall drying time may be as little as 10 or 15 hoursalthough longer times may be utilised if desired.

The improved fire resistant performance of the inorganic intumescentlayers formed in the processes of the present invention means thatrelatively thin intumescent layers may be sufficient to achieve anequivalent fire resistant performance. In the processes of thisinvention the depth of the intumescent layer upon the surface of theglass may be reduced from 1.4 mm to 1.0 mm and more preferably 0.7 mm.The quantity of water which needs to be evaporated in order to producethe dried intumescent layer is correspondingly reduced and the timerequired to dry the solution is also reduced. However even in theseembodiments it is preferred to carry out the initial part of the dryingprocess under carefully controlled conditions and at a relatively slowrate. Attempts to dry the solution more rapidly in this early part ofthe drying process tend to result in the formation of inorganic layershaving inferior properties and in the extreme unacceptable properties.

The waterglass solutions useful in the processes of the presentinvention preferably includes a polyhydric organic compound. The use ofsuch polyhydric compounds is known in the art. Typically the polyhydriccompound will be selected from the group comprising the glycols,glycerine or a derivative of glycerine or a sugar. The most preferredorganic polyhydric compound is glycerol. Preferably the waterglasssolutions (prior to the drying step) will comprise from 2.0 to 10.0% byweight of polyhydric compound.

The waterglass solutions useful in the processes of the presentinvention are preferably prepared in a manner which avoids theproduction of any local extremes of pH value. Conveniently the salt maybe dissolved in an aqueous solution of the polyhydric organic compound.The resulting solution may then be added slowly and with good stirringto a waterglass solution to form a solution which is useful in theprocesses of this invention.

The inorganic intumescent layers may conveniently be produced byspreading the waterglass solution onto the surface of a sheet of glassand subsequently evaporating water from the solution. In order toproduce an inorganic intumescent layer of the desired thickness upon theglass it is necessary to provide an edge barrier on the glass comprisedfor example from clay-like materials which will retain the waterglasssolution during the evaporation step. Such techniques are well known inthe art. The evaporation of water from the waterglass solution ispreferably carried out by drying it in an oven at a temperature of from70° C. to 105° C. for a period of up to 24 hours.

The rate of evaporation of the water may conveniently be controlled byvarying the relative humidity in the atmosphere. By maintaining a veryhigh relative humidity (up to 100 RH) during the initial part of thedrying step the rate of drying may be maintained at a relatively lowlevel. Later in the process the Relative Humidity may be reduced inorder to increase the rate of drying.

When the evaporation is complete the coated glass sheet may be removedfrom the oven and the retaining borders removed by cutting the edgesfrom the sheet. The product is a glass sheet having an inorganicintumescent layer upon one surface thereof. These sheets can beconveniently cut to size and formed into laminated glass units byfurther laminating processes.

The glass sheets with the intumescent coatings may be formed intolaminates by adding a second glass sheet. This second sheet may beuncoated, in which case the product is a laminate having a singleintumescent layer.

Alternatively the second glass may also be coated with an inorganicintumescent layer. It is possible to produce laminates having more thanone intumescent layer which may be useful where enhanced fire resistanceis required.

The use of sheets having relatively thin intumescent layers represents apreferred embodiment of the production of laminates because thelaminated glass unit may be thinner than the conventional units and yetgive equivalent fire resistant performance.

EXAMPLE 1

A transparent fire resistant glazing was made as follows:—

A pane of soda-lime glass 3 mm thick had a moisture and heat resistantbarrier applied around its circumference to form a 5 mm high dam.

Tripotassium citrate (30 parts) was dissolved in deionised water (30parts) and stirred to form a clear solution. This was mixed withglycerol (87% Aqueous solution, 69 parts) and stirred until homogenous.This solution was poured into a sodium silicate solution (38.1% Aqueous,901 parts) with vigorous stirring.

The resultant solution was poured onto the aforementioned pane at adistribution of 5 kg m⁻².

The solution was dried in an oven at 100° C. Steam was injected into theoven until relative humidity reached 100%. The oven was closed and thesolution dried for 14 hours to a residual water content of 20→30%, and athickness of about 1.4 mm. The dried edge barrier,was removed and asecond pane of soda-lime glass bonded to the silicate layer.

This bonded laminated was fire tested under the conditions specified inBS.476 PT. 20-22. The-laminate exceeded 60 minutes integrity asspecified in the standard, at which time the furnace was switched off.

By way of comparison a laminate was manufactured in an identical manner,except that tripotassium citrate was not added to the glycerol.

Under identical test conditions this laminate failed after 37 minutes.

1. A process for the production of an intumescent layer upon the surfaceof a glass substrate which comprises spreading a solution of an alkalimetal waterglass upon the surface of the glass and evaporating waterfrom that solution until an inorganic layer is formed, wherein thewaterglass solution comprises an alkali metal salt of carbonic acid oran α-hydroxy carboxylic acid.
 2. A process according to claim 1, whereinthe α hydroxy carboxylic acid is selected from the group comprisingcitric acid, malic acid, tartaric acid, glycolic acid and lactic acid.3. A process according to claim 1, wherein the waterglass solution whichis spread onto the surface of the glass has a pH in the range 10.5 to11.8.
 4. A process according to claim 1, wherein the pH of thewaterglass solution falls by no more than 2 units of pH during the first5 hours of the drying step.
 5. A process according to claim 1, whereinthe waterglass solution comprises from 0.1 to 2.0% by weight of thesalt.
 6. A process according to claim 1, wherein the waterglass solutioncomprises a polyhydric compound.
 7. A process according to claim 6,wherein the polyhydric compound is a glycol, glycerin or a derivative ofglycerin or a sugar.
 8. A process according to claim 7, wherein thepolyhydric compound is glycerol.
 9. A process according to claim 1,wherein the glass substrate having the waterglass solution spread on thesurface thereof is placed in a heated oven to dry.
 10. A processaccording to claim 1, wherein the water content of the inorganicintumescent layer formed thereon, is in the range 30 to 35% by weight.11. A process according to claim 1, wherein the inorganic intumescentlayer is from 0.3 to 5.0 mm thick.